Objective: a) To perform in vitro studies of granulocytes prepared by different methods for transfusion into patients and, b) to determine the efficacy of granulocyte transfusions as supportive therapy in addition to antibiosis in the management of sepsis in leukopenic patients. Approach: Granulocytes have been separated from heparanized blood of normal donors using the technique of continuous flow filtration leukopheresis (CFFL). CFFL cells are then tested in vitro for their ability to respond to chemotactic stimuli, to adhere to surfaces, to phagocytize and kill bacteria, and to perform a variety of different oxygen-dependent metabolic functions relevant to the bactericidal process. The modification of these processes by ABO incompatible leukagglutinin positive and recipient serum is also under investigation. Leukopenic patients with presumed sepsis are assigned randomly to receive antibiotics and granulocytes or antibiotics alone as management and are compared to survival and other parameters of clinical response.